FIG. 5 shows an example of a conventional high-frequency plasma generating apparatus. FIG. 6 is a cross-sectional view showing a reaction chamber 1 of the same apparatus. The high-frequency plasma generating apparatus shown in these figures can be used for production of a thin film of amorphous silicon semiconductor for solar cells.
The inside of the reaction chamber 1 shown in FIG. 5 and FIG. 6 is equipped with a ladder-shaped electrode 2 as a discharge electrode and a ground electrode 3. The reaction chamber 1 is made gastight, to which a gas supply pipe and an exhaust pipe (both of which are not shown in the drawings) are open at appropriate positions respectively. Through the gas supply pipe, which communicates with a gas supply source, a gas for film formation is introduced into the reaction chamber 1. The exhaust pipe communicates with the suction side of a vacuum pump. The reaction chamber 1 can be evacuated to an internal pressure of about 1 ×10−6 Torr using this vacuum pump.
The ground electrode 3 and the ladder-shaped electrode 2 are disposed opposite to each other at a predetermined distance (for example, a distance of 20 mm). The ground electrode 3 is equipped with a mechanism for holding a glass substrate 4 as a processing object and has a heater built in so as to heat the glass substrate 4. The ladder-shaped electrode 2 needs to be larger than the glass substrate 4 and is a rectangle with the dimensions 1.25 m by 1.55 m when the glass substrate 4 is a rectangle with the dimensions 1.1 m by 1.4 m.
A gas diffusion port of the gas supply pipe is open desirably behind the ladder-shaped electrode 2 (i.e., on the opposite side to the glass substrate 4). Gas is supplied preferably in parallel from several positions.
As shown in FIG. 5, the ladder-shaped electrode 2 is formed by assembling a plurality of parallel longitudinal electrode rods 6 and a pair of transverse electrode rods 7 and 8 into the form of a lattice, and the ladder-shaped electrode 2 is disposed in parallel with and opposite to the glass substrate 4, which is held by the ground electrode 3. Each of transverse electrode rods 7 and 8 of the ladder-shaped electrode 2 is provided with eight feeding points 9. Feeding points 9 of the transverse electrode rod 7 are individually connected to an electric power divider 11, and feeding points 9 of the transverse electrode rod 8 are individually connected to an electric power divider 12. The electric power dividers 11 and 12 are connected to impedance matchers 13 and 14, respectively, by coaxial cables. The impedance matchers 13 and 14 are connected to RF (high-frequency) electric power supplies 15 and 16, respectively. The RF electric power supply 15 is connected to an output portion of an oscillator 17. The RF electric power supply 16 is connected via a phase modulator 21 to the output portion of the oscillator 17. The phase modulator 21 is a circuit which modulates the phase of an output signal S from the oscillator 17 according to output from a sine wave (or triangle wave) oscillator 18, and outputs the modulated signal to the RF electric power supply 16. Here, the output amplitude of the oscillator 18 is constant, and therefore, the phase shift Δθ of modulation by the phase modulator 21 is constant.
With the above structure, the glass substrate 4 on which an a-Si thin film is to be formed is placed on the ground electrode 3 which is set at a temperature of 200° C., for example, then SiH4 gas is introduced at a flow rate of 2 slm, for example, from the gas supply pipe, and the exhaust rate of the vacuum pump system which is connected to the vacuum exhaust pipe is regulated, so as to adjust the pressure inside the reaction chamber 1 to, for example, 40 Pa (300 mTorr). Then, a high-frequency signal of 60 MHz generated by the oscillator 17 is amplified using the RF electric power supplies 15 and 16, and is applied to the transverse electrode rods 7 and 8 of the ladder-shaped electrode 2 through the impedance matchers 13 and 14 and the electric power dividers 11 and 12. This operation generates plasma between the glass substrate 4 and the ladder-shaped electrode 2. At this point, the impedance matchers 13 and 14 are adjusted so that the high-frequency electric power can be efficiently supplied to the plasma generating part. In the plasma, SiH4 is decomposed, and an a-Si film is formed on the surface of the glass substrate 4. An a-Si film with a required thickness can be formed by continuing this film forming operation in this condition for, for example, about 5 to 10 minutes.
However, there is a drawback to the above-described conventional high-frequency plasma generating apparatus in that it is difficult to uniformly form a large area film. This is because a standing wave is generated mainly due to a reflected wave which occurs at an end of an electrode or the like, since the wavelength of the high-frequency wave is about on the same order as the sizes of the electrodes 2 and 3. For example, the wavelength for 60 MHz on the ladder-shaped electrode would be about 3 m. Although the wavelength for 60 MHz in a vacuum is 5 m, the wavelength in plasma is shortened due to increase in capacitance. This wavelength of 3 m gives a ¼ wavelength of about 0.75 m as opposed to the electrode length of 1.25 m, creating maximum and minimum voltage points on the electrode. Therefore, plasma becomes nonuniform following the voltage distribution on the electrode, causing as a result a problem in that a film is formed nonuniformly.
In order to solve such a problem, there are apparatuses as in Japanese Patent Application No. 2001-133830, and there are apparatuses as disclosed in Japanese Patent Application, First Publication (Kokai), No. 2001-274099.
Since an apparatus according to Japanese Patent Application No. 2001-133830 employs a method of high-speed switching between a single-frequency plasma and a double-frequency plasma, plasma changes discontinuously, and a strong plasma is formed in the vicinity of feeding points. Accordingly, there is a limit to obtaining uniformity in the distribution of film thickness, and when a strong plasma exists, the possibility arises that generation of nanoclusters degrades the quality of the film.
On the other hand, an apparatus as disclosed in Japanese Patent Application, First Publication (Kokai), No. 2001-274099 is an apparatus as shown in FIG. 5 with which the phase of the high-frequency applied to the transverse electrode rods 7 and 8 is periodically varied. That is, the phase of output from the RF electric power supply 16 is periodically varied with respect to the phase of output from the RF electric power supply 15 (see FIG. 4 and paragraphs 0091 to 0096 of that publication).